On the hills around Missoula are numerous straight, horizontal ledges called "lap marks" that mark the shorelines as the level of the lake filled to different heights and drained. Glacial Lake Spokane and Glacial Lake Missoula are indicated on the map (above) in medium blue. Glacial Lake Missoula, the largest lake, covered some 3000 square miles and was about 2000 feet deep. The lakes and rivers of today are dark blue.

Glacial Lake Missoula may have looked something like this photo at right. The mountains would have been covered with deep snow and the lake would be rarely as calm as in this photo. The lake would have been the size of Lake Superior and have influenced its own local weather pattern.

Pressure created as the water backed up behind the ice dams would cause them to break and release hundreds of cubic miles of water to flood areas downstream. Jokulhlaups, as these floods are called, still occur in glaciated regions today although not on such a huge scale.

Floating icebergs more massive than these at left carried huge boulders on their backs until they ran aground and melted. The boulders would then become stranded far away from where they formed just as granite rocks from Montana were taken to the Willamette Valley in Oregon during the floods. Geological evidence indicates that 89 or more such floods happened over the years as the ice dam blocked the Clark Fork, broke and re-blocked the river's flow. There is more on the effects of these floods on Page 2a, Spokane to Davenport-2.

Geology Terms

Here are some basic terms used in the tour. Find more geology terms in the
Glossary.

Bar

A mound of gravel and sand deposited by flowing water. Bretz and other geologists identified many large bars in the Channeled Scablands.

Basalt

Volcanic rock caused by partial melting of the Earth's crust.

Channel

The deepest part of a river or bay.

Channeled Scabland

Area in Washington state where huge floods made channels in a large, deep basalt flow. Named by J Harlan Bretz during the 1920's in various publications.
See also Channeled Scablands.

Current Ripple

Mark left on streambed from water current usually less than an inch high and a few inches between the tops (crests) of each ripple. The giant ripples from Lake Missoula floods are as much as 35 feet high and several hundred feet between. See also Ripple Mark.

Erosion

Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.

Erratic

Large rock or boulder carried by water or glaciers and left behind.

Esker

A narrow, winding ridge made of gravel usually formed by streams flowing on a glacier or in a tunnel below the glacier or ice sheet.

Geomorphology

The study of the changes in landforms due to volcanoes, earthquakes, weather, floods, etc.

Gravel

Rounded rock fragments larger than sand.

Ice Age

A period in Earth's history when much of the continents are covered with ice sheets and glaciers.

Jokulhlaups

A flood created when a body of water held by a glacial dam breaks through the confining walls. The Lake Missoula Floods were jokulhlaups.

Moraine

Deposits of rocks, boulders, gravel and sand (called glacial drift) left behind by glaciers as they melted. Terminal moraines are at the end of a glacier.

Outwash

Deposits of rocks, boulders, gravel and sand (called glacial drift) left behind by flowing water from glaciers as they melted.

Outwash Plain

A smooth plain covered by deposits from water flowing from glaciers.

Pleistocene

The period of geologic time that began about two or three million years ago and ended approximately 8,000 years ago.

Ripple Mark

See also Current Ripple. Parallel, elongated mounds of sediment formed in wind or water currents. Large ripple marks found in the Channeled Scablands were the most convincing evidence for the Missoula Floods.

Sediment

Collection of sand, silt, gravel and organic material that sinks to the bottom of a river, lake or ocean. Some or all of these materials may be present.

Till

Deposit from a glacier of unsorted rocks, boulders, gravel and sand (glacial drift).

Varves

Layers of light and dark sediment on the floor of a glacial lake. Each light and dark pair indicates a year since light deposits are from rock dust from melting ice in summer and dark layers are from organic matter.